Advanced turbojet engines now operate at temperatures of approximately 700.degree. C. at the outlet of the high pressure compressor. The high temperatures make it mandatory to provide some means for optimizing the clearance between the rotor and the stator of the high pressure compressor in order to compensate for the differential thermal expansion between these elements. The clearance between the rotor and the stator is typically maintained at a minimum in order to maximize the efficiency of the compressor. However, due to the high operating temperatures, some means must be provided to compensate for the differential thermal expansion.
The compensation devices set forth in U.S. Pat. No. 4,543,039 to Ruis et al and French Pat. No. 2,534,982 consists of supports to attach segments defining an inner casing of the compressor to the outer casing of the compressor. The radial clearance between the rotor blades and the sealing rings of the stator, and between the stator vanes and the rotor seals is controlled by directing a flow of hot or cold air against the supports and/or against the outer casing. These devices are effective if the coefficient of thermal expansion of the support material or of the outer casing is sufficiently large to enable it to respond to temperature changes in the injected air. However, this is typically not the case, and these devices have not completely obviated the clearance problem. Also, these devices suffer the drawback of increased loss of air due to the changing geometry of the support caused by the heat exchange with an air flow.
Mechanical regulators are also known, as illustrated in British Pat. Nos. 2,068,470 and 2,108,591. In these systems, the sealing rings for the rotor blades are provided on the stator, which consists of a plurality of segments circumferentially arranged about the rotor wheel. The segments have cooperating slots and tongues defined on their edges to provide for the required sealing and to allow the segments to move. In U.S. Pat. No. 2,068,470 to segments are circumferentially moved by a gear drive mechanism and, due to the interengagement of projections and a cam track, this circumferential movement also causes the segments to move radially. In an alternative embodiment, the segments are caused to circumferentially move via a pair of link rods extending from a drive ring.
In U.S. Pat. No. 2,108,591 the sealing segments are fixed to one end of a bell crank arm, the opposite end of which is drivingly connected to a control bar. The control bar, which may drive three or more bell cranks, is driven by an actuating cylinder having an expandable and contractible piston rod. Thus, through the bell crank arms, the circumferential displacement of the control bars causes the bell cranks to pivot, thereby imparting a radial displacement to the sealing segments.
These devices are typically mounted between two casings forming the housing surrounding the compressor which entail forming many openings which could possibly lead to substantial leaks. In addition, these devices are mechanically complex which inherently reduces their reliability and increases the risk of malfunction.